leds: populate the device's of_node

If initialization data is available and its fwnode is actually a
of_node, store this information in the led device's structure. This
will allow the device to use or provide OF-based API such (devm_xxx).

Signed-off-by: Jean-Jacques Hiblot <jjhiblot@ti.com>
Signed-off-by: Pavel Machek <pavel@ucw.cz>